The present invention relates to an optical processor.
At present, an optical processor capable of processing the highly multiplexed optical signal is required as one of indispensable elements for successfully constructing a future ultra-high speed optical communication system having a large capacity. In the circumstances like this, an optical gate device reported to the IEEE by M. Suzuki et al might be deemed useful for complying with the above requirement. Their report entitled xe2x80x9cNew Applications of a Sinusoidally Driven InGaAsP Electro-absorption Modulator to In-line Optical Gates with ASE Noise Reduction Effect, xe2x80x9d was published by IEEE, J. Light Wave Technol., 1992, vol. 10, No. 12, pp. 1912-1918.
In the optical gate device disclosed by the above report, there is used an electro-absorption modulator (referred to as xe2x80x9cEA modulatorxe2x80x9d hereinafter) as an optical gate means capable of absorbing and modulating the light of a wavelength used in the optical communication system. The EA modulator has such an optical transmission characteristic that in the vicinity of zero volt, the optical transmission rate does not show any change with respect to the forward voltage applied to the EA modulator but it shows a monotonous decrease in respect of the reverse voltage applied to the same.
In the prior art optical gate device described in the above report, the voltage which is made by superimposing the sinusoidal voltage of a certain repetitive frequency on the DC biased voltage, is applied to the EA modulator in the vicinity of zero volt, thereby attaining such an optical transmission characteristic that the optical transmission rate changes along a rectangular waveform. In the optical gate device of the above report, demultiplexing of RZ (return-to-zero) optical pulse signal is carried out such that the timing of the maximum peak (xe2x80x9cmountain topxe2x80x9d) of the sinusoidal voltage is synchronized with the timing of the signal train taken out as the optical output.
According to the above report, as the RZ signal train formed by alternately multiplexing the RZ signal of a bit rate of Gbit/s with respect to the time axis, is given to the EA modulator which is sinusoidally driven at the repetitive or repetition frequency of 5, GHz, every other RZ signal of 5, Gbit/s is extracted from the RZ signal train passing through the EA modulator, thereby enabling another RZ signal train formed of the remaining (i.e. not extracted) RZ signals to be extracted. This way of using the optical gate device is specially referred to as xe2x80x9coptical demultiplexing.xe2x80x9d Also, the alternately multiplexing means the time-division multiplexing of the signal trains belonging to different series.
As explained above, the greatest feature of the optical gate device discussed in the above report is that the optical gate waveform of a rectangular shape can be formed with ease by applying the sinusoidal voltage to an EA modulator. Furthermore, the other feature of the above optical gate device is that the sinusoidal voltage is externally applied to the EA modulator, so that it is possible to make the period of the optical gate waveform variable, and thus different from the mode synchronization method. Still further, the other feature of the above optical gate device is that it is not always needed for the feeder line to have a wide-band characteristic as the modulation voltage is the sinusoidal one.
However, some problems to be solved are still left in the demuliplexing system or the optical gate system, which employs the above-mentioned optical gate device. Thus, those will be discussed in the following.
(1) First Problem
The first problem is that the optical demultiplexing system wherein the signal is extracted every other signal from the signal train alternately multiplexed with respect to the time axis, is not only less efficient in making use of the optical energy but also tends to increase the power consumption by the system and to enlarge the system scale as well.
In the optical demultiplexing system employing the above optical gate device, the alternately multiplexed signal train is branched into two parallel branches before executing the optical demultiplexing operation. Then, signals are extracted every other signal from the first signal train of the one branch, and the similar extraction is performed on the second signal train of the other branch. At this time, the phase of the second sinal train is shifted by a half period with respect to the first one. Therefore, this system comes to waste a half of the optical energy, thus being less efficient in making use of the optical energy.
In this case, still further, in order that the optical signal train can hold the same optical power for the sensitivity of the corresponding receiver before and after being processed by the optical demultiplexing, the optical power of the alternately multiplexed signal train has to be doubled by amplifying it at a certain stage either before or after the signal train being branched into two parallel branches. As a result, the power consumption and the scale of the system can not help being increased.
In association with the above first problem, when the above prior art optical gate device processes the optical signal train that is multiplexed with a multiplex factor of n (n: integer), it will encounter the following two significant problems.
One problem that exists is that it can not be expected that the operation of the EA modulator will be stable over a long period of time. In the optical demultiplexing operation of the alternately multiplexed signal train, the DC bias voltage is generally set in the vicinity of the OFF-state region. However, when the signal train is n-multiplexed, the DC bias voltage has to be deeply set in the OFF-state region. Consequently, while the high frequency voltage is deflected toward the negative side, there is applied to the EA modulator the resultant large negative bias voltage obtained as a sum of the DC bias voltage and the high frequency amplitude voltage. As the result of this, the light absorption at the end face for receiving the incident light on the EA modulator is exponentially increased, so that there is a high possibility that the element breaks down and it becomes difficult to expect the stable operation of the EA modulator over a long period of time.
The other problem exists in that the amount (nxe2x88x921)/n of the energy of the input optical signal is unavoidably wasted. In order to remove this problem thus far, the optical signal power has to be amplified to be n times as much as that of the incident light at a certain stage either before or after the signal train is branched into parallel branches of n. As a result, the power consumption and the scale of the system can not help being increased.
(2) Second Problem
The second problem is that in the demuliplexing system or the optical gate system employing the above prior art optical gate device, the gate width (time duration) can be changed only in a narrow range. The ratio of the gate width and inverse value of the repetitive frequency is generally called a duty ratio or cycle. In the gate system employing the above optical gate device, an obtainable duty ratio is in the range of utmost 0% through. 60%. In order to obtain the duty ratio of 60% or more, it is needed to set the DC bias voltage as a large positive voltage or to make the amplitude of the high frequency voltage zero volt and set the DC bias voltage as zero volt or more.
However, if the DC bias voltage exceeds the built-in voltage of the pin junction in the EA modulator, the abrupt current begins to flow, which causes the heat generation at the pin junction, spotaneous light emission, absorption of free carriers, etc. On the other hand, in order to make the amplitude of the high frequency voltage zero, it is need to additionally provide a new electric circuit therefor.
The present invention has been made in view of the above described and other problems in connection with the prior art optical gate device and other optical processors.
According to the invention, in order to solve the problems as described above, there is provided a novel and improved optical processor which includes: one optical modulator of the electro-absorption type or two or more optical modulators of the same type which are optically connected in series with each other; at least one voltage generation means which is electrically connected with the one or more optical modulators and generates the modulation voltage to be supplied to the one or more optical modulators; and at least one electric current detection means which is electrically connected with the one or more optical modulators and detects the extinction current or the light absorption current generated by the optical modulator.
In the above optical processor, the voltage generation means drives the optical modulator with the modulation voltage generated thereby. The optical modulator can absorb and modulate the input light in response to the modulation voltage. The electric current detection means can detect the extinction current which is generated in the optical modulator at the time of absorbing and modulating the input light. In other words, according to the above structure, it becomes possible to execute both of the strength modulation and the detection of the input light.
Optical connection in the above optical processor can be achieved, for instance by means of optical cascade arrangement of optical elements thereof, integrating them on an identical substrate, a simple optical alignment and so on. The optical cascade arrangement is a kind of method for making the optical connection used in case of packaging elements to an optical module. For instance, according to this method, each core of optical waveguides of tow or more optical elements are connected with each other by using an optical transmission means such as an optical fiber or a planar lightwave circuit (referred to as xe2x80x9cPLCxe2x80x9d hereinafter), thereby forming an optical passage. In case of integrating two or more optical elements on an identical substrate, they are connected with each other through an optical waveguide used in common or are coupled with a directional optical coupler intervening therebetween. Furthermore, in case of simply aligning two or more optical elements, they are coupled with each other by means of an optical coupler such as a lens, so that their optical axes are arranged so as to meet each other.
The optical processor according to the invention may further includes at least one relay means which relays the electrical connection between the voltage generation means and the optical modulator and also the electrical connection between the electric current detection means and the optical modulator, and which supplies the modulation voltage generated by the voltage generation means, to the optical modulator and at the same time, supplies the extinction current generated in the optical modulator to the electric current detection means.
The above relay controls the direction of supplying the modulation voltage and the extinction current. Thus, it is possible to minimize or remove the effect of the modulation voltage to the electric current detection means, and also as the effect of the extinction current to the voltage generation means.
The above relay means may be constructed in the form of an electric circulator or a filter acting on the wavelength of the electric signal. Furthermore, it is preferable that the optical modulator has a triode structure. The triode structure is defined as the waveguide structure of an optical element which is formed by crystallographically joining two semiconductor regions of the identical conductive type through one semiconductor region of the opposite conductive type together. The triode structure is explained in detail in JP unexamined patent publications (KOKAI) Nos. 9, (1997)-301301, and 10, (1998)-030477, in the names of the inventors of this patent application, which are herein incorporated by reference.
As described in the above two publications, when adopting the triode structure as the waveguide structure for the optical elements, the range of driving voltage can be made far wider compared with that of the waveguide structure using a pn or pin junction (referred to as xe2x80x9cdiode structurexe2x80x9d hereinafter). Therefore, the optical modulator including the triode structure can have a large degree of freedom and an excellent controllability with respect to the modulating operation. For instance, in the case of applying the superimposed voltage formed of the bias voltage and the sinusoidal voltage to the optical modulator having the triode structure, the bias voltage can be deeply set while the amplitude of the sinusoidal voltage can be set widely, so that the duty ratio of the modulating operation by the optical modulator can be controlled in the range of 0, through 100%.
Furthermore, according to the invention, in order to solve the problems as described above, there is further provided a novel and improved optical processor which includes: one optical modulator of the electro-absorption type or two or more optical modulators of the same type which are optically connected in series with each other, any of the optical modulators having a triode structure; and at least one voltage generation means which is electrically connected with the one or more optical modulators and generates the modulation voltage to be supplied to the one or more optical modulators. Thus, there is provided the optical processor having the function as an optical gate which makes use of the optical modulator with the triode structure showing the excellent controllability in respect to the operation.
Still further, according to the invention, in order to solve the problems as described above, there is further provided a novel and improved optical processor wherein the optical signal can be extracted from a time-division multiplexed optical signal train at a predetermined time interval and the other optical signal can be added to that optical signal train at the same time interval, the optical processor including one optical modulator of the electro-absorption type or two or more optical modulators of the same type which are optically connected in series with each other, all of the optical modulators having a triode structure and extracting the optical signal. Thus, there is provided the optical processor having an optical add/drop function which makes use of the optical modulator with the triode structure which shows the excellent controllability in respect of the operation.
The triode structure of the optical modulator may includes a pinip junction or an nipin junction, and a pinp junction or an nipn junction.
In the optical processor according to the invention, there may be further provided a light detection means which is optically connected with the last stage of a sequence of said optical modulators. With the structure like this, the optical processor comes to have a light receiving function capable of receiving the input light by means of both of the optical modulator and the light detection means.
In the optical processor having two or more optical modulators according to the invention, the operation timing of respective optical modulators can be set to be different from one another. With the structure like this, the optical processor comes to be able to perform desired processing such as modulating, gating, or detection of the time-division multiplexed optical signal on the basis of the time-division channel thereof.
The optical processor according to the invention can include two or more optical modulators and further includes a means for adjusting the operation timing of modulators, which shifts the operation timing of one optical modulator from that of the other.